The playboy son of one of Africa's most notorious dictators commissioned plans for a luxury superyacht costing $380m (£234m) – nearly three times his country's combined health and education budgets, according to a corruption watchdog.

Teodorin Obiang, eldest son of Teodoro Obiang, the president of Equatorial Guinea, wanted to build the world's second most expensive yacht after the Russian tycoon Roman Abramovich's $1.2bn Eclipse, the campaign group Global Witness said. It condemned the plan as "outrageous extravagance" in a country where, despite vast oil wealth, 20% of children die before their fifth birthday and few people live beyond 50.

President Obiang has ruled for more than 30 years and been accused of grave human rights violations. Forbes magazine estimated his wealth at around $600m.

Teodorin Obiang, 41, is his minister of agriculture and apparently being groomed as his father's successor. Global Witness said his "extravagant lifestyle" includes a $35m mansion in Malibu , a $33m jet and a fleet of luxury cars – yet his ministerial salary is $6,799 a month.

Global Witness claimed Obiang asked the German company Kusch Yachts to build the superyacht under the codename "Project Zen". It said Kusch employees told investigators the 387ft yacht would have a cinema, restaurant, bar, pool and a $1.3m security system complete with floor motion sensors, photoelectric barriers and fingerprint door openers. The basic design was completed by Kusch in December 2009 for €250,000 (£212,192) with an original delivery date set for late next year.

Gavin Hayman, the Global Witness director of campaigns, said: "Kusch's motto is, 'We don't just build yachts that you use, we create a dream that you live'. But this sounds more like a nightmare for Equatorial Guinea's oppressed and brutalised citizens, whose money may once again be put to Teodorin's benefit rather than theirs."

Kusch Yachts declined to comment.

Karin Jorgensen, secretary to the managing director, said: "As you will know, everybody working in the business of mega yachts is obliged by contract to strict confidentiality, as far as projects are concerned, and also with respect to clients."

A spokeswoman for the Equatorial Guinea government said: "The minister requested a sketch of the design of a yacht three to four years ago and no contract or agreement exists with the Kusch company.

"If there is no agreement or contract, it is evident that one cannot talk about cost, because there was no major interest in the proposal received and this proposal was not implemented.

"The minister did not order that a yacht be built in order to supposedly pay for it. The minister requested a sketch of what building a yacht would entail, and then dismissed the idea of buying it.

Therefore, there was no agreement or formal order to build the yacht; and if it had been viable to purchase it, he would have bought it with income from his private business activities and not with funds derived from sources of illegal financing or corruption."

In 2006 Obiang junior justified his wealth in a sworn affidavit to a South African court questioning his ownership of luxury mansions and cars. He said officials in his country were allowed to join with foreign companies bidding for government contracts and this means "a cabinet minister ends up with a sizable part of the contract price in his bank account."

A US justice department investigation into US banks accepting some $75m from Teodorin said in a 2007 report that "it is suspected that a large portion of Teodoro Nguema Obiang's assets have originated from extortion, theft of public funds, or other corrupt conduct".

The writer Juan Tomás Ávila Laurel is in the 17th day of a hunger strike demanding justice for the people of Equatorial Guinea, inspired by the popular revolutions in Egypt, Libya and Tunisia.

The Obiang family and President Denis Sassou-Nguesso of Congo-Brazzaville are facing a fraud inquiry in Paris for allegedly embezzling state funds to acquire vast assets in France, including bank accounts, villas and luxury cars.